CH4 Oxidation In Soils Research Articles

Fluxes of CO2and CH4in soil profiles of a mountainous watershed of Nepal as influenced by land use, temperature, moisture and substrate addition

Effects of land use, moisture, temperature and substrate on production of CO2 and consumption of CH4 were measured in a series of laboratory incubation experiments on bulk soil samples from 0–10, 10–20, 20–40, 40–60, 60–80 and 80–100 cm soil depths under four dominant land uses [forest, grazing land, irrigated rice on level terraces (Khet), and upland maize–millet on sloping terraces (Bari)] of Mardi watershed (area=144 km2), Nepal. In addition, baseline physical and chemical properties of these soils were measured. The production of CO2-C day−1 per unit soil organic carbon (SOC) content in topsoil was lowest in grazing land, indicating a possibility of higher C sequestration with this land use than with other land uses. There was a decreasing trend of CO2 emission with soil depth in all land uses, as was also the case with the SOC content. The CO2 emission was increased by 90, 58, 27 and 23% for Bari, Khet, grazing and forest soil, respectively, with increase in moisture level from 40 to 60% (w/w). The CO2 release from forest soil went up from 15 to 50 μg CO2 g−1 dry soil with increase in temperature from 5 to 15 °C and it further increased to 67 μg CO2 g−1 at 20 °C with estimated Q10 values of 3.4 and 1.8, respectively. Significantly higher amounts of CO2 were emitted from all the land use types upon addition of glucose to the soil, indicating high potential of microbial activity. Consumption of CH4 was more rapid in the soil from 10 to 20 cm depth for all the land use types. There was a 89% increase in CH4 consumption in forest soil with increase in moisture level from 40 to 60%, while it was decreased by 38, 73, and 40% for Khet, Bari and grazing soil, respectively. Addition of (NH4)2SO4 inhibited CH4 oxidation in soils of all land uses, indicating a negative effect of N fertiliser input on CH4 uptake in soil.

Pristine New Zealand forest soil is a strong methane sink

AbstractMethanotrophic bacteria oxidize methane (CH4) in forest soils that cover ∼30% of Earth's land surface. The first measurements for a pristine Southern Hemisphere forest are reported here. Soil CH4 oxidation rate averaged 10.5±0.6 kg CH4 ha−1 yr−1, with the greatest rates in dry warm soil (up to 17 kg CH4 ha−1 yr−1). Methanotrophic activity was concentrated beneath the organic horizon at 50–100 mm depth. Water content was the principal regulator of (r2=0.88) from the most common value of field capacity to less than half of this when the soil was driest. Multiple linear regression analysis showed that soil temperature was not very influential. However, inverse co‐variability confounded the separation of soil water and temperature effects in situ. Fick's law explained the role of water content in regulating gas diffusion and substrate supply to the methanotrophs and the importance of pore size distribution and tortuosity. This analysis also showed that the chambers used in the study did not affect the oxidation rate measurements. The soil was always a net sink for atmospheric CH4 and no net CH4 (or nitrous oxide, N2O) emissions were measured over the 17‐month long study. For New Zealand, national‐scale extrapolation of our data suggested the potential to offset 13% of CH4 emissions from ca. 90 M ruminant animals. Our average was about 6.5 times higher than rates reported for most Northern Hemisphere forest soils. This very high was attributed to the lack of anthropogenic disturbance for at least 3000–5000 years and the low rate of atmospheric nitrogen deposition. Our truly baseline data could represent a valid preagricultural, preindustrial estimate of the soil sink for temperate latitudes.

Advances of study on atmospheric methane oxidation (consumption) in forest soil

Next to CO2, methane (CH4) is the second important contributor to global warming in the atmosphere and global atmospheric CH4 budget depends on both CH4 sources and sinks. Unsaturated soil is known as a unique sink for atmospheric CH4 in terrestrial ecosystem. Many comparison studies proved that forest soil had the biggest capacity of oxidizing atmospheric CH4 in various unsaturated soils. However, up to now, there is not an overall review in the aspect of atmospheric CH4 oxidation (consumption) in forest soil. This paper analyzed advances of studies on the mechanism of atmospheric CH4 oxidation, and related natural factors (Soil physical and chemical characters, temperature and moisture, ambient main greenhouse gases concentrations, tree species, and forest fire) and anthropogenic factors (forest clear-cutting and thinning, fertilization, exogenous aluminum salts and atmospheric deposition, adding biocides, and switch of forest land use) in forest soils. It was believed that CH4 consumption rate by forest soil was limited by diffusion and sensitive to changes in water status and temperature of soil. CH4 oxidation was also particularly sensitive to soil C/N, Ambient CO2, CH4 and N2O concentrations, tree species and forest fire. In most cases, anthropogenic, disturbances will decrease atmospheric CH4 oxidation, thus resulting in the elevating of atmospheric CH4. Finally, the author pointed out that our knowledge of atmospheric CH4 oxidation (consumption) in, forest soil was insufficient. In order to evaluate the contribution of forest soils to atmospheric CH4 oxidation and the role of forest played in the process of global environmental change, and to forecast the trends of global warming exactly, more researchers need to studies further on CH4 oxidation in various forest soils of different areas.

Nitrogen fertilization of wheat residue affecting nitrous oxide and methane emission from a central Ohio Luvisol

Fertilization of wheat (Triticum aestivum, L.) residue applied to degraded soils has shown promise as an option to restoring soil organic C (SOC) stocks, but the impact of the practice on N2O and CH4 emissions is not clear. It was hypothesized that, in addition to the mulch-induced soil wetness conditions favorable for N2O and CH4 formation, emission of these gases will be stimulated due to increased availability of mineral N and interference of NH4+ with CH4 oxidation in soils. During the period February–November 2000, fluxes of N2O and CH4 were monitored in a plant-free central Ohio Crosby soil (fine, mixed, mesic Aeric Ochraqualf) amended for 4 years with wheat straw (bare, 0; low, 8 Mg ha–1 year–1; and high, 16 Mg ha–1 year–1) without and with N fertilization (244 kg N ha–1). The average annual N2O fluxes were 1.1 kg N2O-N ha–1 in the unfertilized and 4.1 kg N2O-N ha–1 in the fertilized treatments. Annual N2O emission (Y, mg N2O-N m–2) was strongly correlated to the maximum daily flux (X, mg N2O-N m–2 day–1; Y=48.3X−58.1, R2=0.85, P<0.001) recorded on experimental plots. These flux maxima occurred at spring thaw in the unfertilized, and 6–30 days after fertilization in the fertilized treatments. Net CH4 uptakes were measured on some occasions; overall, however, all the treatments were net CH4 emitters with annual rates of 3.6, 4.9 and 5.1 kg CH4-C ha–1 in the bare, low and high residue treatments, respectively. No significant effect of fertilization and mulch rate on CH4 fluxes was found, but temperature and landscape position appeared as strong controllers. Regardless of treatments, the highest CH4-emitting plots were located in a minor depressional area at the experimental site. A comparison of SOC gain and N2O and CH4 emission expressed as CO2-equivalents indicates that the residue treatments have a net CO2-mitigating effect, but since C sequestration rates are expected to decrease with time, that positive effect will likely vanish after 7 and 12 more years in the fertilized and unfertilized residue treatments, respectively.

Methane fluxes on agricultural and forested boreal organic soils

Abstract. Annual methane fluxes from an organic soil in eastern Finland, originally drained and planted with birch (Betula pendula) and then later cultivated, were studied for two years using a chamber technique. The agricultural soils growing grass or barley or without vegetation, generally acted as sinks for CH4. Surprisingly, the agricultural soils emitted CH4 during a warm dry summer. The CH4 oxidation capacity and CH4 uptake rate of the forested site was three times that of agricultural soils. Also, the forest soil better retained its capacity to take up CH4 during a dry summer. Despite periods of CH4 emission, the agricultural soils were annual sinks for CH4, with uptake rate of CH4‐C varying from 0.1 to 3.7 kg ha−1 yr−1. The forested soil had a methane uptake rate of 3.9 kg CH4‐C ha−1 yr−1. All the soils acted as sinks for CH4 during winter, which contributed up to half of the annual CH4 uptake. The capacity of soils to transport gases did not explain the larger CH4 uptake rate in the forest soil. At the same gas filled porosity, the forest soil had a much larger CH4 uptake rate than the agricultural soil. Neither the soil acidity (pH 4.5 and 6.0) nor high ammonium content appeared to limit CH4 uptake. The results suggest that CH4 oxidation in agricultural organic soil is more sensitive to soil drying than CH4 oxidation in forested organic soil.

Differential inhibition of CH4 oxidation in bare, bulk and rhizosphere soils of dryland rice field by nitrogen fertilizers

Summary Laboratory experiments were conducted to elucidate the variation of CH4 oxidation pattern in three different soils (rhizosphere, bulk and bare) of a dryland rice (Oryza sativa L. cultivar Narendra-118) field. The rhizosphere soil exhibited strongest CH4 oxidation activity and the bare soil the weakest. Control soils (no N-fertilization) exhibited higher capacity for CH4 oxidation than N-fertilized soils. Above conclusions were supported by measurements of MOB (methane oxidizing bacteria) population size, NH4+-N concentration (as a potential inhibitor of CH4 oxidation), soil moisture content and kinetic parameters (Km and Vmax) for the same conditions. MOB population size was significantly higher in the rhizosphere (548.6 × 105 cells g−1) than bulk (459.1 × 105 cells g−1) and bare (38.3 × 105 cells g−1) soils. The MOB population size was highest in control (418.8 × 105 cells g−1) followed by NH4Cl (359.2 × 105 cells g−1) NH4NO3 (323 × 105 cells g−1), and urea (293.7 × 105 cells g−1) treated soils. NH4+-N concentration was the highest in bare soil (10.5 μg g−1) followed by bulk (8.8 μg g−1) and rhizosphere (6.8 μg g−1) soil. Urea treated soil had maximum NH4+-N (10.1 μg g−1) and control soil the minimum (6.3 μg g−1). Apparent half saturation constant (Km) and maximum oxidation rate (Vmax) increased significantly from bare to bulk to rhizosphere in control as well as fertilized soil. Km ranged from 4.79 to 139.50 μg g−1 dry soil, and Vmax from 0.04 to 0.60 μg h−1 g−1 dry soil. These differences in kinetic parameters might be due to differential species composition of CH4 oxidizing community and/or conditioning of MOB to different soil microenvironments. This study has demonstrated competitive inhibition effect of NH4+-N on CH4 oxidation, which is relatively less strong for rhizospheric MOB due to root associated processes. Es wurden Laborexperimente durchgefuhrt, um die Variation des CH4-Oxidationsmuster bei drei verschiedenen Boden (Rhizosphare, offen, bearbeitet) eines Reisfeldes (Oryza sativa L. cultivar Narendra-118) im Trockenanbau zu erhellen. Der Rhizospharenboden entfalteten die starkste CH4-Oxidationsaktivitat, offene Boden die geringste. Die Kontrollboden (ohne N-Dungung) zeigten eine hoher Kapazitat fur die CH4-Oxidation als die N-gedungten Boden. Die oben genannten Schlussfolgerungen wurden durch die Messungen der Populationsgrose der MOB (methanoxidierende Bakterien), der Bodenfeuchte und der kinetischen Parameter (Km und Vmax) unter gleichen Bedingungen gestutzt. Die MOB-Populationsgrose war signifikant groser bei den Rhizospharenboden (548.6 × 105 Zellen g−1) als bei den bearbeiteten (459.1 × 105 Zellen g−1) und offenen Boden (38,3 × 105 Zellen g−1). Die MOB-Populationsgrose in den Boden war am hochsten bei den Kontrollen (418.8 × 105 Zellen g−1), gefolgt von den mit NH4Cl (359.2 × 105 Zellen g−1), mit NH4NO3 (323 × 105 Zellen g−1) und den mit Harnstoff (293.7 × 105 Zellen g−1) behandelten Boden. Die NH4+-N-Konzentration war bei den offenen Boden am hochsten (10.5 g μg−1), gefolgt von den bearbeiteten (8.8 μg g−1) und den Rhizospharenboden (6.8 μg g−1). Der mit Harnstoff behandelte Boden hatte maximalen (10.1 μg g−1) und der Kontrollboden minimalen (6.3 μg g−1) NH4-N. Die Michaelis-Konstante (Km) und die maximale Oxidationsrate (Vmax) nahmen vom offenen uber den bearbeiteten bis zum Rhizospharenboden sowohl bei den Kontrollen als auch den gedungten Boden signifikant zu. Km variierte von 4.79 bis 139.50 μg g−1 trockener Boden und Vmax von 0.04 bis 0.60 μg g−1 trockener Boden. Diese Unterschiede in den kinetischen Parametern konnten auf die differentielle Artenzusammensetzung der CH4-oxidierenden Lebensgemeinschaft und/oder die Konditionierung der MOB auf unterschiedliche Bodenmikroumwelten zuruck zu fuhren sein. Diese Studie hat einen konkurrierenden, hemmenden Effekt von NH4+-N auf die CH4-Oxidation gezeigt, der aufgrund der mit den Wurzeln assoziierten Prozesse relativ weniger stark fur rhizospharische MOB gilt.

Combined assessment of methane oxidation and nitrification: an indicator of air-borne soil pollution?

Soil nitrification and CH4 oxidation activity were measured in six forested soils in a region of south-western Poland differently affected by heavy metals and hydrocarbon contamination from a coke production plant, road traffic and general urban sources. A multivariate analysis of data from 80 sites within the six forested areas investigated identified Ni and an unidentified hydrocarbon (H1) as negatively affecting nitrification and CH4 oxidation in a model (P=0.02) that also included soil organic matter and pH. Univariate statistical analysis of these two suspected toxicants demonstrated a negative influence of Ni on nitrification (P<0.05) within the two areas in which soil Ni concentrations were highest. In addition, it revealed a negative influence of H1 on CH4 oxidation (P<0.06) at the two sites with the highest soil concentrations of H1 among those sites where CH4 oxidation occurred. The distribution of nitrification and CH4 oxidation were inversely related, as would be expected from their contrasting responses to soil disturbances and inorganic N levels. Among sites where at least one of the two processes occurred, Ni or H1 levels were high in only 18% of them, which is significantly lower than the frequency of high pollutant concentrations (67%) among sites where neither nitrification nor CH4 oxidation occurred (P<0.05). The apparent ubiquity with which at least one of the two processes occurs in unpolluted soil samples suggests that combined measurement of nitrification and CH4 oxidation could serve as a useful pollution indicator compared with processes which are selected more randomly for toxicological studies.

Numerical Model for Biological Oxidation and Migration of Methane in Soils

Concern over the potentially negative impacts of climate change has resulted in a search for techniques to reduce anthropogenic emissions of methane, a primary greenhouse gas. Microbial oxidation of methane in soils may serve as an inexpensive technique for reducing CH4 emissions from sources such as landfills and heavy oil wells. To gain a better quantitative understanding of the biological and physical processes limiting CH4 oxidation in soils and biofilters, a numerical reactive-transport model was developed. The model inputs include CH4 source strength, soil bulk density, moisture content, and biological kinetic parameters. The outputs consist of gas concentration profiles, CH4 oxidation rates, and surface flux rates. A series of soil column and batch incubation experiments were performed on a variety of soil types to calibrate and verify the model. The model was also verified by reproducing experimental results found in the literature.

Methane Oxidation in Three Alberta Soils: Influence of Soil Parameters and Methane Flux Rates

Current concern over the potentially negative impacts of climate change has brought attention to anthropogenic sources of methane, a primary greenhouse gas. Two such emission sources are methane leakage at heavy oil wells and sanitary landfills. At both of these sources, substantial quantities of methane could potentially be oxidised by methanotrophic microbes living in soils. Optimisation of this phenomenon may serve as an inexpensive technique for reducing methane emissions. Soil column and batch incubation experiments were performed on a landfill loam, an agricultural loam and a sedge peat to gain a better quantitative understanding of the biological and physical processes limiting CH4 oxidation in soils that undergo the freeze-thaw cycles associated with northern climates. Moisture content emerged as a critical variable that can limit a soil's CH4 oxidation potential. For example, the oxidation rate of the agricultural soil was seen to increase by an order of magnitude after increasing its moisture content from 6% to 10% of its dry weight.

Assessment of CH4 and N2O fluxes in a Danish beech (Fagus sylvatica) forest and an adjacent N-fertilised barley (Hordeum vulgare) field: effects of sewage sludge amendments

Fluxes of CH4 and N2O were measured regularly in an agricultural field treated with 280 g m−2 of sewage sludge. In a nearby beech forest N2O and CH4 fluxes were measured in a well-drained (dry) area and in a wet area adjacent to a drainage canal. We observed brief increases of both CH4 and N2O emissions immediately following soil applications of digested sewage sludge. Cumulated values for CH4 emissions over the course of 328 days after sludge applications indicated a small net source in sludge treated plots (7.6 mg C m−2) whereas sludge-free soil constituted a small sink (-0.9 mg C m−2). The CH4 emission amounted 0.01% of the sludge-C. Extrapolated to current rates of sludge applications in Danish agriculture this amounts to 0.1% of the total agricultural derived CH4. Sludge applications did not affect cumulated fluxes of N2O showing 312 mg N2O–N m−2 and 304 mg N m−2 with and without sludge, respectively. Four months after the sludge applications a significant effect on CO2 and NO emissions was still obvious in the field, the latter perhaps due to elevated nitrification. Nitrous oxide emission in the beech forest was about six times smaller (45 mg N m−2) than in the field and independent of drainage status. Methane oxidation was observed all-year round in the forest cumulating to -225 mg C m−2 and -84 mg C m−2 in dry and wet areas. In a model experiment with incubated soil cores, nitrogen amendment (NH4Cl) and perturbation significantly reduced CH4 oxidation in the forest soil, presumably as a result of increased nitrification activity. Sludge also induced net CH4 production in the otherwise strong CH4 oxidising forest soil. This emphasises the potential for CH4 emissions from sewage sludge applications onto land. The study shows, however, that emissions of N2O and CH4 induced by sewage sludge in the field is of minor importance and that factors such as land use (agriculture versus forest) is a much stronger controller on the source/sink strengths of CH4 and N2O.

Effect of rice plants on CH4 production, transport, oxidation and emission in rice paddy soil

To understand the integrated effects of rice plants (variety Wuyugeng 2) on CH4 emission during the typical rice growth stage, the production, oxidation and emission of methane related to rice plants were investigated simultaneously through laboratory and greenhouse experiments. CH4 emission was significantly higher from the rice planted treatment than from the unplanted treatment. In the rice planted treatment, CH4 emission was higher at tillering stage than at panicle initiation stage. An average of 36.3% and 54.7% of CH4 produced was oxidized in the rhizosphere at rice tillering stage and panicle initiation stage, respectively, measured by using methyl fluoride (MF) technique. In the meantime, CH4 production in the planted treatments incubated under O2-free N2 condition was reduced by 44.9 and 22.3%, respectively, compared to unplanted treatment. On the contrary, the presence of rice plants strongly stimulated CH4 production by approximately 72.3% at rice ripening stage. CH4 emission through rice plants averaged 95% at the tillering stage and 89% at the panicle initiation stage. Based on these results, conclusions are drawn that higher CH4 emission from the planted treatment than from unplanted treatment could be attributed to the function of rice plants for transporting CH4 from belowground to the atmosphere at tillering and panicle initiation stage, and that a higher CH4 emission at tillering stage than at panicle initiation stage is due to the lower rhizospheric CH4 oxidation and more effective transport mediated by rice plants.

Influence of Heavy Metals on Methane Oxidation in Tropical Rice Soils

In a laboratory incubation study, the effect of select heavy metals on methane (CH4) oxidation in two rice soils was investigated under two moisture regimes. Heavy metals differed in their effect on CH4 oxidation in both soils under the two water regimes. Cr significantly inhibited CH4 oxidation in the alluvial soil at 60% moisture holding capacity, while Cu stimulated the process. On the contrary, Zn inhibited CH4 oxidation in both alluvial and laterite soils only under flooded conditions. Application of rice straw alleviated the inhibitory effect of heavy metals on CH4 oxidation and CO2 production. Inhibition of CH4 oxidation in the alluvial soil was related to the methanotrophic bacterial population in Cr- and Zn-amended alluvial soil.

Effects of soil moisture and temperature on CH4 oxidation and N2O emission of forest soil

Soil samples were taken from depth of 0–12 cm in the virgin broad-leaved/Korean pine mixed forest in Changbai Mountain in April, 2000. 20 μL·L−1 and 200 μL·L−1 CH4 and N2O concentration were supplied for analysis. Laboratory study on CH4 oxidation and N2O emission in forest soil showed that fresh soil sample could oxidize atmospheric methane and product N2O. Air-dried soil sample could not oxidize atmospheric methane, but could product N2O. However, it could oxidize the supplied methane quickly when its concentration was higher than 20 μL·L−1. The oxidation rate of methane was increased with its initial concentration. An addition of water to dry soil caused large pulse of N2O emissions within 2 hours. There were curvilinear correlations between N2O emission and temperature (r2=0.706, p<0.05), and between N2O emission and water content (r2=0.2968, p <0.05). These suggested temperature and water content were important factors controlling N2O emission. The correlation between CH4 oxidization and temperature was also found while CH4 was supplied 200 μL·L−1 (r2=0.3573, p<0.05). Temperature was an important factor controlling CH4 oxidation. However, when 20 μL·L−1 CH4 was supplied, there was no correlation among CH4 oxidization, N2O emission, temperature and water content.

Effects of prolonged soil drought on CH4 oxidation in a temperate spruce forest

Our objective was to determine potential impacts of changes in rainfall amount and distribution on soil CH4 oxidation in a temperate forest ecosystem. We constructed a roof below the canopy of a 65‐year‐old Norway spruce forest (Picea abies (L.) Karst.) and simulated two climate change scenarios: (1) an extensively prolonged summer drought of 172 days followed by a rewetting period of 19 days in 1993 and (2) a less intensive summer drought of 108 days followed by a rewetting period of 33 days in 1994. CH4 oxidation, soil matric potential, and soil temperature were measured hourly to daily over a 2‐year period. The results showed that annual CH4 oxidation in the drought experiment increased by 102% for the climate change scenario 1 and by 41% for the climate change scenario 2, compared to those of the ambient plot (1.33 kg CH4 ha−1 in 1993 and 1.65 kg CH4 ha−1 in 1994). We tested the relationships between CH4 oxidation rates, water‐filled pore space (WFPS), soil matric potential, gas diffusivity, and soil temperature. Temporal variability in the CH4 oxidation rates corresponded most closely to soil matric potential. Employing soil matric potential and soil temperature, we developed a nonlinear model for estimating CH4 oxidation rates. Modeled results were in strong agreement with the measured CH4 oxidation for the ambient (r2 = 0.80) and drought plots (r2 = 0.89) over two experimental years, suggesting that soil matric potential is a highly reliable parameter for modeling CH4 oxidation rate.

Quantifying Methane Oxidation from Landfills Using Stable Isotope Analysis of Downwind Plumes

Landfills are major contributors to the atmospheric CH4 budget. A major uncertainty in estimating CH4 flux from landfills is determining the attenuation of CH4 emission by methanotrophic bacteria in the aerobic outer portions of the cover soil. These bacteria intercept the gas as it migrates toward the atmosphere. To estimate cover soil oxidation, we made seasonal measurements of the difference in the δ13C of CH4 within the anoxic zone and CH4 released from landfills and captured downwind at two landfills in the Northeastern United States. Anoxic zone CH4 at the Nashua Landfill averaged −54.6 ± 0.7‰, n = 205, and displayed no significant seasonal pattern. Methane was in excess over ambient air concentrations in the downwind plume ranging from 2.13 to 3.41 ppmv. The δ13C of excess CH4, as determined by mass balance with correction for ambient air CH4, varied from −49.35 to −54.28‰. We used these values to calculate soil CH4 oxidation, which ranged from 0 to 23.6%. Oxidation was greatest in the summer and in the fall, with an annual value of 12 ± 8%. At a second landfill, plume CH4 ranged from 1.96 to 2.92 ppmv with excess δ13C values of −52.17 to −58.06‰. Oxidation at this landfill ranged from 0 to 14%.

Short-term effects of nitrogen on methane oxidation in soils

The short-term effects of N addition on CH4 oxidation were studied in two soils. Both sites are unfertilized, one has been under long-term arable rotation, the other is a grassland that has been cut for hay for the past 125 years. The sites showed clear differences in their capacity to oxidise CH4, the arable soil oxidised CH4 at a rate of 0.013 μg CH4 kg–1 h–1 and the grassland soil approximately an order of magnitude quicker. In both sites the addition of (NH4)2SO4 caused an immediate reduction in the rate of atmospheric CH4 oxidation approximately in inverse proportion to the amount of NH4 + added. The addition of KNO3 caused no change in the rate of CH4 oxidation in the arable soil, but in the grassland soil after 9 days the rate of CH4 oxidation had decreased from 0.22 μg CH4 kg–1 h–1 to 0.13 μg CH4 kg–1 h–1 in soil treated with the equivalent of 192 kg N ha–1. A 15N isotopic dilution technique was used to investigate the role of nitrifiers in regulating CH4 oxidation. The arable soil showed a low rate of gross N mineralisation (0.67 mg N kg–1 day–1), but a relatively high proportion of the mineralised N was nitrified. The grassland soil had a high rate of gross N mineralisation (18.28 mg N kg–1 day–1), but negligible nitrification activity. It is hypothesised that since there was virtually no nitrification in the grassland soil then CH4 oxidation at this site must be methanotroph mediated.

Effect of the organic horizon on methane oxidation and uptake in soil of a boreal Scots pine forest

We studied in situ and in the laboratory whether the organic horizon (O horizon) could act as a physical ‘buffer system’ against soil dryness and water stress for the CH4 oxidizing microbes in boreal coniferous forest soil. The laboratory experiments with samples from a Scots pine forest soil showed that the CH4 oxidation took place in the uppermost mineral soil and was not affected by removal of the O horizon (peeling) and irrigation. The removal of a thin O horizon (2–3 cm) in early June increased the mean summertime uptake of CH4 by 50% and decreased the release of CO2 by 50%. Thus, the O horizon acts as a diffusion barrier, lowering the uptake rate of atmospheric CH4. The experimental 50% increase in precipitation by irrigation (from 58 to 84 mm month−1) had only a marginal effect on the gas fluxes in both control plots and peeled plots. During the study summer two dry periods with low matric potentials (<−1 MPa) in mineral soil occurred without there being any reduction in CH4 uptake. In laboratory experiments, the gravimetric water content of 12% (−9 kPa) led to the maximum CH4 oxidation. The CH4 oxidation was still 60% of the maximum at a water content of 4% (<−1.5 MPa) and totally inhibited at a water content of 1%. The results reveal that a low soil water content is not an important factor in restricting CH4 oxidation in mineral soil of boreal forests.

The influence of land use and pesticides on methane oxidation in some Belgian soils

In a first experiment, the effect of land use on the uptake rate of atmospheric CH4 was studied in laboratory incubations of intact soil cores. A soil under deciduous forest showed the highest CH4 oxidation. Its overall CH4 uptake during the measuring period (202 days) was 1.03 kg CH4 ha–1. Natural grassland showed the second highest CH4 oxidizing capacity (0.71 kg CH4 ha–1). The overall amount of CH4 uptake by fertilized pasture was 0.33 kg CH4 ha–1. CH4 oxidation in arable soils with different fertilizer treatments varied between 0.34 and 0.37 kg CH4 ha–1. Undisturbed soils had a higher CH4 uptake capacity than agricultural soils. The moisture content of the soil was found to be an important parameter explaining temporal variations of CH4 oxidation. Different methods of fertilization which had been commenced 10 years previously were not yet reflected in the total CH4 uptake rate of the arable soil. In a second experiment, a number of frequently used pesticides were screened for their possible effect on CH4 oxidation. In a sandy arable soil lenacil, mikado and oxadixyl caused significantly reduced CH4 oxidation compared to the control. Under the same conditions, but in a clayey arable soil, mikado, atrazine and dimethenamid caused a reduction of the CH4 uptake. In a landfill cover soil, with a 100-fold higher CH4 oxidation rate, no inhibition of CH4 oxidation was observed, not even when the application rate of pesticides was tenfold higher than usual.

Wetting stimulates atmospheric CH4 oxidation by alpine soil

Studies were done to assess the effects of soil moisture manipulations on CH4 oxidation in soils from a dry alpine tundra site. When water was added to these soils there was a stimulation of CH4 oxidation. This stimulation of CH4 oxidation took time to develop. One to three days after water additions no stimulation was observed. Nine days after rewetting, CH4 oxidation was greatly stimulated. This time delay suggests that methanotrophs grew in response to water additions, or that they take a long time to recover from metabolically inactive resting stages.

Sugarcane fields: sources or sinks for greenhouse gas emissions?

The quantities of greenhouse gases emitted into the atmosphere from sugarcane fields, and their contribution to the total emissions from Australian agriculture, have never been estimated with any degree of certainty. This review was conducted to collate the available information on greenhouse gas emissions from the Australian sugarcane crop. Estimates were made for the emissions of the 3 major greenhouse gases―carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O)―from known or suspected sources. Sinks for the sequestration of the gases also have been identified. CO2 was found to be emitted during burning of the crop and from trash-blanketed and bare sugarcane fields. Total emissions from these sources in the 1994 season were estimated at 7·6 Mt CO2-C/year. However, the sugarcane crop was identified as a major sink for C, with uptake by the crop in 1994 estimated at 13· 4 Mt CO2-C/year. N2O emanating from sugarcane soils via denitrification following application of fertiliser accounted for 45-78% of total gaseous N emissions. Estimates of N2O emissions from all land under sugarcane in 1994 totalled 4·4 kt N2O-N/year from denitrification with a further 6·3 kt N2O-N emitted from areas that are still burnt. This review suggests changes in management procedures that should limit the opportunities for denitrification in the soil and thus reduce N2O emissions. Methane evolution occurs during the smouldering phase, following burning of the crop, with production estimated at 6·7 kt CH4-C/year in 1994. CH4 oxidation in soil was identified as an important process for removal of atmospheric CH4, as were trash-blanketed soils. Although these figures are our best estimate of gaseous production from sugarcane fields, there still remains a degree of uncertainty due to sampling variability and because of the extrapolation to the entire sugarcane area. However, the coupling of new laser techniques with known micrometeorological methods will allow for a more precise sampling of greenhouse gas emissions over a larger area. Estimates would thus be more representative, resulting in a greater degree of confidence being placed in them by the sugar industry.